Chemical Vapor Deposition (CVD) methods are commonly used for deposition of electronic and optoelectronic materials and devices. For example, Metal Organic Chemical Vapor Deposition (MOCVD) is an epitaxial deposition technology used to form the compound semiconductor materials and hetero-structures from which laser diodes, light-emitting diodes, transistors, etc. are fabricated. In the MOCVD process, gaseous chemical precursors pass over a heated crystalline substrate seed crystal, where a pyrolytic reaction causes chemical decomposition and subsequently produces solid films. In the prototypical case of GaAs growth, the precursors arsine (AsH3) and trimethylgallium ([CH3]3Ga) are pyrolyzed to yield the Ga and As species required for growth of the GaAs film. For growth of other compound semiconductors and alloys, analogous mixtures of other group-III organometallic and group-V hydride precursors are blended into the gas stream. Oftentimes the precursor chemicals used in CVD processes are liquids or solids, whose vapors are transported into the deposition chamber by a carrier gas. In the case of MOCVD, group-III metal organic (MO) sources are supplied as liquids or solids, contained in stainless-steel vessels, or bubblers. Among the challenges associated with the use of liquid or solid precursor chemicals in CVD is determining the amount of precursor remaining in the bubbler; and similarly, when the metal organic (MO) source in the bubbler is fully consumed. This is difficult because the source is virtually inaccessible, being contained within an opaque steel container.
Existing techniques to determine the amount of precursor remaining in the bubbler have a number of drawbacks. One technique is to use the deposition reactor's control system to calculate, integrate, and record the amount of transported material (assuming a saturated mixture); and this feature is implemented on many commercial MOCVD reactors. In most cases, it is necessary to remove the bubbler from the system and weigh it to determine the quantity of remaining MO. However, this is undesirable because it involves breaking seals and thus exposes the gas lines to air contamination. Other techniques use fiber optic probes, capacitance-based probing, whereby the steel bubbler itself forms one plate of a capacitor, and a rod inserted into the center of the bubbler comprises the other plate, and the MO liquid is the dielectric between them; and ultrasonic detection where an ultrasonic transducer bounces sound off the liquid surface to determine its exact height. However, these techniques are expensive, requiring complex electronics and parts. In addition, they are not so easily applied to solid precursors, such as the commonly used trimethylindium, biscyclopentadienylmagnesium, and carbonterabromide. Another option is an instrument that directly measures the OM vapor concentration in the carrier gas, based on measuring the speed of sound in the gas mixture. This method offers good control of gas mixture compositions; but for most situations it is not useful for determining the fill level of a bubbler.